Engine deceleration control typically focuses on providing acceptable vehicle deceleration control while attempting to minimize transmission lash concerns. For example, various approaches are available for transitioning control from the vehicle operator to idle speed operation during deceleration and gear changes of the transmission.
However, the inventors herein have recognized that when trying to control torque to a desired value, both airflow and spark timing are variables that can affect the torque output. As such, to maximize fuel economy, it would be desirable to provide the desired torque by reducing airflow while keeping spark timing at an optimal value. On the other hand, there may be limitations to the minimum amount of airflow that provides a desired level of combustion smoothness.
Thus, in one example embodiment, a method of operating an engine of a vehicle having a powertrain is provided that includes: during a driver tip-out condition where the vehicle is decelerating, reducing airflow to reduce engine torque output without reducing torque by retarding spark timing, following the driver tip-out, until airflow reaches a minimum value below which combustion may be degraded; and upon airflow reach said minimum value, retarding spark timing to further reduce engine output.
In this way, desired torque trajectories can be obtained while improving fuel economy when possible, and yet avoiding degradation of combustion.
In other embodiments, further improvements in deceleration control can be achieved by additionally providing an improved transition to idle speed operation and reduced engine dips.